Such a nacelle is generally constituted by several composite panels produced in two parts and then assembled together. The shape of the panels is computed by aerodynamic specialists so as to limit the drag adjacent the air foil. As a general rule, it is usually curved and of large size. During production of each panel, it is important to keep this form so as to preserve the shapes and dimensions.
So as to rigidify the shape and to guarantee the dimension of these panels, local stiffening is used.
Different techniques are used at present.
The panel can be stiffened by a continuous cellular structure, covering all the surface of the panel. This cellular structure is sandwiched between two layers of fibers or pre-impregnated plies.
In the case of a single curing, during production of such a panel, upon placing under vacuum, the plies of the layer of pre-impregnated fibers have the tendency to sink into the open cells of the cellular structure. Thus, before the polymerization temperature of the resin is reached, the consistency of these fabrics is soft. At the end of the cycle, the panel thus formed comprises hollows which modify the aerodynamic shape.
To overcome these problems, there exist two solutions:
using a cellular structure with small cells such that the plies of the layer of fibers cannot sink into them. The principal drawback is the weight of the panel produced, because the smaller the size of the cells, the greater the density of the structure,
producing the panel in several steps. First, the layer of pre-impregnated fibers is cured, because once hardened, the layer cannot sink into the cells of the cellular structure. Upon the second curing, there is added the cellular structure and the last layer. This process requires producing the panel in several steps requiring a fairly long production time, as well as a large number of different tools.
Moreover, this technique has the drawback of being difficult to repair by the user, once the panel becomes damaged. In case of shock, moisture may seep into the cellular structure, degrading the structural properties of said structure and requiring a very precise repair according to the very strict rules.
Another technique consists in using local stiffeners, in the form of strips of cellular structure or foam strips. The use of a cellular structure gives rise to the same drawbacks, although reduced, as in the preceding technique, namely: sinking of the plies of fibers, the weight of the panel, the difficulty of repair, the need for several polymerization cycles, and the risk of moisture uptake as a result of shock.
Foam has the advantage of avoiding the pre-impregnated plies of fibers sinking in, because it has no open cells. It nevertheless has a major drawback: the plies of pre-impregnated fibers sink during cooling of the panel because of the difference of coefficient of expansion between said fibers and the foam. This problem exists moreover also with cellular structure. Thus, the materials used to produce stiffening strips (foam, cellular structure, . . . ) have in general a coefficient of expansion that is greater than the pre-impregnated materials. In the autoclave, at the polymerization temperature, the plies of pre-impregnated material solidify whilst the material of the stiffening strip dilates. During cooling of the piece, said material retracts to return to its initial position (before heating) and carries with it the solidified composite material, causing sinking. As a result, slight hollows appear on the panel, which modifies the fluid flow and generates unacceptable aerodynamic drag. Moreover, these hollows weaken the panel by generating the onset of flexure. In this case, the foam strip or structure necessarily has a structural role to compensate this force. This requires strict repair rules.
It is possible to avoid the use of a cellular structure by replacing it with localized shaped stiffeners. These localized stiffeners can have different shapes (Z, U, I, . . . ) as a function of the place where they are positioned. They are first made of a composite material, by the draping/molding technique, on tools specific to their shape. There must accordingly be as many tools as shapes to be produced. The production of the panel takes place in two steps: curing of the layer of pre-impregnated fibers, then cementing of the localized stiffeners.
This cementing operation requires very precise tools, because there exist problems of adaptation between the stiffeners and the panels, namely problems of contraction at the points of contact. This technique generates risks of loosening of the stiffeners, in addition to multiple polymerization operations.
Thus, in the present economic context, in which the highest quality is sought at the best price, the preceding techniques although used in industry have not been entirely satisfactory.